fix: Remove x-default alternate links entry for non-root pathnames when using localePrefix: 'always'

[amannn]

Closes #799

Before:

link: <https://example.com/en/about>; rel="alternate"; hreflang="en",
      <https://example.com/de/about>; rel="alternate"; hreflang="de",
      <https://example.com/about>; rel="alternate"; hreflang="x-default"

After:

link: <https://example.com/en/about>; rel="alternate"; hreflang="en",
      <https://example.com/de/about>; rel="alternate"; hreflang="de"

Notes:

• Only non-root pathnames are affected, the root (/) will still include the x-default entry.
• The x-default entry is considered optional and we're still providing all links to localized pages, therefore this is fine from an SEO perspective.

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